Gram-negative animal and plant pathogens use the type III secretion system (TTSS) to infect virulence factors (called effectors) directly into the cytoplasm of their host cells and the extra-cellular environment. Some effectors of plant pathogens elicit plant defenses while others interfere with theme. The plant pathogen system of Arabidopsis thaliana and Pseudomonas syringae (both organisms have been sequenced) offers the unique possibility to easily and economically manipulate both host and pathogen. This system can be used to identify in straightforward and rapid assays the effectors that interfere with plant defense pathways. This will be achieved by expressing effectors in plants and determining their ability to interfere with defense responses by assaying reporter gene activity and bacterial growth. To determine if effectors sufficient to alter plant defenses contribute to pathogen fitness, these effectors will be deleted from OP. syringae. Growth of mutant strains will be compared with growth of wild-type stains in plants to determine fitness. Using micro array analysis it will be possible to assign changes in gene transcription during disease to the action of individual effectors. The long-term goal is to dissect plant defense pathways further by identifying the molecular components of defense pathways including the molecular plant effector molecules.